In its most strictly construed definition, quicksand is a mixture
of sand and water that exhibits thixotropic behavior.

"Thixotropic behavior" is a term from rheology (the science of fluid
flow) meaning that a pseudoplastic fluid exhibits a time-dependent decrease
in viscosity.

A "pseudoplastic" fluid is a non-Newtonian fluid in which the viscosity
decreases as the shear stress increases.

"Viscosity" is the "thickness" or resistance to flow of a liquid. Water
has a very low viscosity (it flows well). Molasses, on the other hand, has
a very high viscosity.

A Newtonian fluid is one (like water or molasses) where the viscosity is
pretty much constant under normal stress. A "non-Newtonian fluid," however,
can change in viscosity in response to stress. Examples of non-Newtonian
fluids include transmission fluid, ketchup, and a mixture of cornstarch
and water.

"Shear stress" refers to stress (pressure) that deforms something
without changing its volume.

So here's what we have:

Regular fluids have a constant viscosity, even under stress.

Pseudoplastic fluids decrease their viscosity based on the
current amount of shear stress applied to them.

Thixotropic fluids decrease their viscosity based on the amount of shear
stress applied and the length of time that the stress is
applied.

The classic example of quicksand's thixotropic behavior occurs when
a progression of vehicles is passing through an area containing quicksand.
The first vehicle is the least likely to have problems, but as each
succeeding vehicle passes, exerting pressure on the quicksand, the
viscosity of the quicksand continues to decrease until it is unable to
support the weight and a passing vehicle will become mired, even if
it is smaller and lighter than those that went before. The same
principle can occur with people travelling in a row, although this is
somewhat less common since people tend to say "hey, this is quicksand,
let's take a different path" unless they are Deep Sinking members.

This most literal kind of "quicksand" is most frequently encountered
in riverbeds and tidal areas where water is pushed through a bed of sand
with enough force to create fluid conditions.

It is important to note that this is not a practical definition, as
quicksand "in the wild" can be subject to a number of conditions that alter
its properties and behavior from the above-described "laboratory
conditions," such as a "crust" of dried sand over the top
of it where water has been evaporated away.

Due to the specific gravity (density) of "real" quicksand, it is
virtually impossible for a person to sink and disappear into it.
Although this can and does happen, such incidents are exceedingly rare
and tend to involve additional weight, such as heavy metal-frame
backpacks. The vast majority of deaths involving quicksand occur
as a result of exposure: a person becomes trapped and unable to
escape and then dies as a result of starvation, cold, or rising
rides.

Furthermore, there are a large number of other substances which are
colloquially referred to as "quicksand" even though they may not be made
of sand and water, and may not display thixotropic behavior at all.
Even "ordinary" mud, if deep enough, is likely to be referred to as
"quicksand" by people who have encountered it. Some of these, such
as bogs containing large amounts of decaying vegetable matter, may
actually be more dangerous than "real" quicksand due to lower
densities.

Please note that there is a lot of bad science on the web
on the subject of quicksand. Don't believe everything you read,
including what's here.

The popular perception of quicksand has been created and sustained
almost entirely by movies and television shows. According to such
sources, quicksand is most often found in jungles and swamps, and
anyone who becomes trapped in it is probably doomed unless they are
the hero or the hero is suitably close at hand.

Since this portrayal is so unrealistic, there is a reactionary
popular view that quicksand is a myth and does not actually exist.
Although it is true that "killer quicksand" (term borrowed from an
episode of "Mythbusters") does not exist (but see above about vegetable
matter bogs), quicksand and similar substances certainly do exist.

For the most part, the Deep Sinking definition of "quicksand" has its
roots in the Hollywood portrayal: a deep, sucking morass that tends to
devour its victims for which the best advice is almost always, "don't
struggle, you'll only sink faster." However, for people on Deep
Sinking, this tends to be just a jumping-off point. (Or, if you
prefer, a "jumping-in" point.)